Sound absorbing system



Feb. 13, 1934. v P. s. CALDWELL 1,947,287

sounn ABSORBING SYSTEM Filed June '7, 1933 Fl'gl.

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Fig. 3.

I'm/enrol" Philip G. Caldwell,

Patented Feb. 13, 1934 PATENT OFFICE SOUND ABSORBING SYSTEM Philip G. Caldwell, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June '7, 1933. Serial No. 674,660 Claims. '(01. 181-30) My invention relates to arrangements of sound absorbing means, particularly to such arrangements for enclosed spaces or rooms such as broadcasting studios, auditoriums and the like, and

5 has for its principal object the provision of sound absorbing or damping means so disposed in rooms of this character that a given amount of damping material is employed to the best advantage.

It has been common practice to control the acoustic conditions of an enclosure such as a broadcasting studio or the like by means of sound absorbing or damping material. Results of acoustical studies have led investigators to believe that the greater the quantity of absorbent material and the less reflecting material present in a room, the more damped the room will be. Heretofore in accordance with this fact a relatively large amount of absorbing material has been employed in the damping of a room, reaoiiecting objects such as furniture being covered with sound absorbents and reflecting walls hung heavily with drapes. I have found, however, that in a given enclosure having sound reflecting walls, sound absorbing or damping equally as efficient that secured by the above means may be secured in accordance with my invention by the use of a considerably smaller amount of sound absorbing material, as will be explained hereinafter.

In setting forth the principles which underlie my invention, a helpful analogy may be drawn between the acoustic phenomena involved and an electric transmission line. The voltage on an eicctric transmission line may be held analogous to the sound pressure in a room and the line current in the transmission line may be held analogous to the velocity with which particles, in a medium through which the sound waves pass, oscillate.

Now the condition existing at any point on a transmission line which is in a steady state condition depends on the load which is placed at the receiver end of the line. The voltage and current, at any point on the line, may be divided into 45- two voltages and two currents: one the voltage and current of an incident decaying wave train, and the other the voltage and current of a reflected decaying wave train. If the reflected decaying wave train is small, the effect of this wave 50 train on the voltage and current distribution along the line is not great. On the other hand, if the reflected decaying wave train is comparable in magnitude to the incident decaying wave train, as it is when the receiver is either open or closed, this reflected wave train will have a great efiect on the voltage and current distributions along the line. Resonances of both voltage and current will be produced. When the receiver is open, the voltage will have maximum values at distances which are even quarter wave lengths away from the open end of the line, and the current will have maximum values at distances which are odd quarter wave lengths away from the open end. Conversely, when the receiver is closed, voltage maxi'ma will occur at odd quarter wave lengths and current maxima at even quarter wave lengths away from the receiver end. When the line is terminated by its characteristic impedance, the incident decaying wave train will be wholly absorbed by the receiver, no reflection will occur, and the voltage and current resonances will not exist.

Consider now a room in which there is a sound source and a reflecting surface in front of the source. This reflecting surface is analogous to the receiver of the above-described transmission line. If the surface could be made to be entirely sound absorbing, so that all the sound impinging upon it would be absorbed, no reflection would occur, and the analogy to the electric transmission line terminated by a characteristic impedance would be perfect.

If, on the other hand, the above-mentioned sound reflecting surface were so highly reflecting that little if any sound energy would be absorbed, 35 which is the case with the great majority of surfaces in a room, then the analogy to an electric transmission line with an open-circuited receiver would be perfect. The sound pressure (corresponding to the voltage in the transmission line) would be a maximum at the reflecting surface, and the velocity of the particles (corresponding to the current in the transmission line) would of necessity be zero.

In conformation, then, of the acoustic phenomena above described to transmission line theory, at substantially a quarter wave length of the sound wave projected against the reflecting surface from this reflecting surface, the sound pressure would be very low and the particle velocity would be maximum. Therefore, since the sound wave absorbing process involves, as is well known, a conversion of sound wave energy to heat energy by means of friction between the oscillating air particles and the particles in the interstices of the absorbing material, a sound absorbent is most effective when placed, in accordance with my invention, substantially a quarter wavelength of the projected sound wave distant from the sound refleeting surface. At that point the air particle velocity is maximum and hence a maximum amount of friction occurs between the air particles and the sound absorbing material.

Since in practice a quarter wavelength for one frequency is a very different distance from a quarter wavelength for some other frequency, the sheet or curtain of absorbing material may be placed at a distance from the sound reflecting surface which is equal to a quarter wavelength for the lowest frequency which will commonly occur in the room. The absorbing material is then highly effective for this frequency, and also very effective for other frequencies which have odd quarter wavelengths occurring at this same distance from the reflecting surface, i. e., for frequencies which are odd harmonics of the base frequency. In addition, the sheet of absorbing material placed as above described will be eflective for all frequencies which are so reflected as to be characterized by appreciable particle velocity when they reach the position of the absorbing material, as will be explained more fully hereinafter.

A second sheet or curtain may be placed intermediate the reflectlng surface or wall and the above described sheet or curtain. It is desirable, further, to place a layer or sheet of sound absorbent material near or at the sound reflecting wall in order that high frequencies, for which a quarter wavelength is a very short distance, may

' also be absorbed.

' velocity is maximum and hence sorbing or damping occurs for My invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. 1 illustrates semidiagrammatically in section a room or like enclosure in which my invention has been embodied, and Figs. 2 and 3 illustrate modifications of the invention.

In Fig. 1 an enclosure 1 which may be an acoustic studio, auditorium, or the like, is shown having a sound reflecting wall 2 adapted to have projected thereagainst a sound wave from a source of sound 3 which in the present embodiment of the invention is illustrated as a loud speaker. In accordance with my invention, in order to damp effectively the reflected sound, a sound absorbing element 4 which may be a sheet or curtain of suitable sound absorbing material is spaced substantially one quarter wavelength of the projected wave distant from the reflecting wall 2, as illustrated by the sound wave curve 5 shown in dotted line. As hereinbefore set forth, at the quarter wavelength point 6 of curve 5 the air particle the friction occurring between the air particles and the sound absorbing material 4 is also a maximum.

It will readily be seen from a consideration of the curve 5 that, while the maximum sound absound waves for which sound absorbing element 4 is one quarter wavelength or multiples thereof distant from reflecting wall 2, nevertheless element 4 will have a marked absorbing effect on any waves which are characterized by appreciable particle velocity at the illustrated position of the absorbing element 4.

As shown in Fig. 2 in order to absorb very short waves in addition to waves absorbed by the sheet or curtain 4, a sound absorbing element '7 is placed at or near the reflecting wall 2. The element 7 may take the form of a layer of suitable sound absorbing material on the wall 2 or a sheet or curtain of suitable material spaced closely adjacent this wall.

As shown in Fig. 3, the sound absorbing means comprising sheet or curtain 4 may further include a second sheet or curtain 8 of absorbing material spaced intermediate the reflecting wall 2 and sheet 4. As in the modification illustrated in Fig. 2, in Fig. 3 a sound absorbing element 7 may be added at or near the reflecting wall.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In an enclosure having a wall adapted to reflect a sound wave projected thereagainst, a. sheet of sound absorbing material spaced from said wall a distance substantially one quarter the Wave length of said projected sound wave.

2. In an enclosure having a wall adapted to reflect sound waves projected thereagainst, a sheet of sound absorbing material spaced from said wall a distance substantially one quarter the wave length of said projected sound wave, and a second sheet of sound absorbing material spaced intermediate the first-named sheet and the wall.

3. In an enclosure having a wall adapted to reflect sound waves projected thereagainst, a sheet of sound absorbing material spaced from said wall a distance substantially one quarter the wave length of said projected sound wave, and a second sheet of sound absorbing material spaced substantially midway between the first-named sheet. and the wall.

4. In an enclosure having a wall adapted to reflect a sound wave projected thereagainst, a sheet of sound absorbing material spaced from said wall a distance substantially one quarter the Wave length of said projected sound wave, and a. layer of sound absorbing material closely adjacent the sound reflecting surface of said wall.

5. In an enclosure having a wall adapted to reflect sound waves projected thereagainst, a sheet of sound absorbing material spaced from said wall a distance substantially one quarter the wave length of said projected sound wave, a second sheet of sound absorbing material spaced intermediate the first-named sheet and the wall, and a layer of sound absorbing material closely adjacent the sound reflecting surface of the wall.

PHILIP G. CALDWELL. 

